In recent years, increase in plastic waste has become a great social problem. Since many of polymeric materials have hitherto been developed and produced in search of high performance and long-term stability, they are not easily decomposed in a natural environment. Therefore, how to dispose and manage a large quantity of plastic waste which has become useless becomes a social problem on a world-wide scale. Of the plastic waste, hollow containers formed from a variety of synthetic resins, such as polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate (PET), and polyvinyl chloride resins become a special problem because they are bulky.
Under the circumstances, biodegradable polymers, which are degraded by natural microorganisms, attract attention as polymeric materials which impose no burden on the environment. The biodegradability can be evaluated by, for example, a degradability test in soil (soil degradability test). Since plastic hollow containers such as PET bottles are required to have, for example, good barrier properties, toughness, heat resistance, melt processability and profitability, however, any plastic hollow container, which fully satisfies these requirements and exhibits biodegradability, has not been yet obtained.
More specifically, hollow containers making use of the conventional biodegradable plastics involve the following problems. For example, hollow containers based on starch are unsatisfactory in water resistance, barrier properties, mechanical properties, heat resistance and mildew resistance and involve a problem that such a plastic material is difficult to melt-process, so that its processing cost becomes high. Hollow containers based on cellulose are unsatisfactory in barrier properties to oxygen and carbon dioxide and mechanical properties and involve a problem that such a plastic material is difficult to melt-process, so that its processing cost becomes high. Hollow containers based on a microorganism-produced polyester are unsatisfactory in barrier properties to oxygen and carbon dioxide and mechanical strength and involve a problem that their production cost is very high. Hollow containers based on a synthetic type polyester such as a polysuccinate (Japanese Patent Application Laid-Open No. 172425/1995) are unsatisfactory in barrier properties to oxygen and carbon dioxide, mechanical strength and heat resistance and involve a problem that succinic acid and butanediol, which are raw materials for the polyester, are considerably expensive.
Hollow containers based on polylactic acid, which is a semi-synthetic type polyester, are unsatisfactory in barrier properties to oxygen and carbon dioxide and mechanical strength. Since L-lactic acid, which is an optically active substance used as a raw material, is required to have a high purity, the hollow containers must be produced through fermentation of a biological process, and there is hence a limit to their production at low cost. Further, since polylactic acid has a high glass transition temperature, Tg, it also involves a problem that it is difficult to compost under ordinary composting conditions.
Biodegradable containers making use of a lactic acid polymer have recently been proposed (Japanese Patent Application Laid-Open No. 23828/1994). As a specific example thereof, a hollow container based on a lactic acid-polyglycolic acid (weight ratio=50:50) copolymer is disclosed (Example 6). However, the copolymer containing a lactic acid component in such a great amount generally loses crystallizability and becomes an amorphous polymer having a glass transition temperature, Tg of about 30-50.degree. C., so that the copolymer involves a problem that it is brittle, low in mechanical strength and extremely low in barrier properties to carbon dioxide and oxygen and heat resistance.
A biodegradable bottle for drippings obtained by molding polylactic acid or a copolymer of lactic acid and glycolic acid in accordance with a blow molding process has been proposed (Japanese Patent Application Laid-Open No. 278785/1994). The bottle for drippings is a small-sized hollow container generally having an internal volume of 5 ml or smaller and is used as a small-lot packaging container for edible seasonings such as soy, sauce, drippings for spitchcock and buckwheat noodle, and liquid spices. In this publication, pellets of a lactic acid-glycolic acid copolymer, which are obtained by melting an artificial fibrous cloth for a synthetic absorbent, tissue substitute ("VICRYL MESH", trade name) produced by Johnson & Johnson Medical Co. at about 200.degree. C. in an extruder into a strand and chopping the strand are used as the copolymer of lactic acid and glycolic acid (Example 1). The lactic acid-glycolic acid copolymer used in this example has a melt viscosity as low as about 300 Pa.s (at a shear rate of 100/sec). Therefore, it is difficult to apply such a copolymer to usual extrusion blow molding to mold it due to too hard drawdown of its melt though it can be formed into small containers such as bottles for drippings. This publication describes the fact that blow molding was conducted at a resin temperature of about 175.degree. C. Even when a parison is blow-molded at such a high resin temperature, only a substantially non-oriented, extrusion blow molded container which scarcely undergoes molecular orientation can be obtained. Such a non-oriented, extrusion blow molded container has poor barrier properties and heat resistance. Besides, such hollow containers of medium size (about 25 ml) or greater become insufficient in mechanical properties.